How to get back to hotel after late concert at Palau Sant Jordi

Just take a taxi, they're cheap, plentiful, and safe even late at night. Just a guess but I'm thinking what you consider late at night is not really late. Remember in Spain we usually have dinner around 10:00 p.m., you'll still see plenty of people out and about by midnight

Do not rent a car!
As Carlos says, even if Elton doesn't take the stage until after 10pm for that concert (listed at 9pm) by the time it is over people will still be eating dinner and strolling around, and taxis will be plentiful.

A concert starting at 9pm in Spain is not considered "late" by our standards. By the time it finishes there´ll be a very lively atmosphere in the city and plenty of things going on. If worried, make sure you arrange a taxi pick-up after the concert in the vicinity of the hotel, in any case it´ll be safe.

Excuse my ignorance... but will this concert last more than 3 hours? I see it announced at 9 pm, and on a weekday (Mon-Thu) the metro operates until 12 am. Furthermore, regarding bus lines, when the day lines finish (between 11 pm and 12 am, depending on the line), the night lines take over until 6am. Depending on where your hotel is located you're likely to have a night bus stop close enough. Then, as mentioned, taxis, are available 24/7 and which will be plenty queuing at Palau Sant Jordi to pick up people leaving the concert. And lastly, with an occupancy of 24,000, the vast majority will head down 'to the city' (mind me, Palau Sant Jordi is located on a hill named Montjuic which is full of stadiums, museums and parks) so you'd be walking along with thousands of people ("safety in numbers"). And there'll be plenty of police too, that's for sure with those crowds.

Thank you all so much... Great information.